Dihydronicotinamide adenine dinucleotide (phosphate) (NAD(P)H) and its oxidized form, nicotinamide adenine dinucleotide (phosphate) (NAD(P)+) are ubiquitous biomolecules associated with cellular energy metabolism in both eukaryotic and prokaryotic organisms.1 It has been reported that the NAD(P)+/NAD(P)H couples are essential cofactors for more than 300 dehydrogenases.2,3 Increased activity of dehydrogenases such as aldehyde dehydrogenases has been reported in various human cancers4 and has been found to interfere with certain chemotherapeutic treatments.5,6 Accordingly, dehydrogenase inhibitors have been developed for the treatment of human diseases,4,5,7 as well as applications in alcohol dependence,8 cocaine addiction,9 anxiety,10 and as resensitizing agents for cancers.11 Thus, the development of sensitive and specific NAD(P)H sensors could not only open numerous possibilities for dehydrogenase characterization but also identify inhibitors of dehydrogenases for the development of novel anticancer agents,12 antibiotics,13 and pesticides.14 
Gold nanoparticles (AuNPs) have been widely used for applications in sensing, catalysis, imaging, diagnostics, therapy and drug delivery due to their unique optical and electronic properties and good biological compatibility.15-21 The properties of AuNPs usually depend on their size and shape,22-26 and the dissolution of gold has proven to be an effective way to resize or reshape these particles.27 Traditionally, colorimetric detection of NADH is based on the growth of gold nanoparticles, which requires a large sample volume, a longer reaction time, as well as sophisticated analytical instruments to confirm detection efficacy. See, for example, PCT Publication WO2006008742 and Chinese Publication CN1821751A.
Therefore, there still remains a need for diagnostic devices and methods that require less sample volume and easier method of fabrication, while maintaining the sensitivity for detecting analytes critical for biological activities in various applications.